Drinking too much water could possably have an effect on your AFI. But you should be drinking somewhere between 2 and 3 litres a day as a minimum and perhaps more on hot days or if you are a very active person. So drinking too much water, for most people, would be difficult.
Lots of salt is not good for anyone and can cause fluid retention. However a small amount of salt in your diet is needed to help keep your muscles working correctly. In most cases you would be able to get the amount of salt needed from the foods you eat without the need to add salt.
Hot weather does not have any effect on your AFI. All pregnant women carry extra fluid. For some women the amount of fluid in your body can increase by as much as 80%. The hot weather can contribute to your feet swelling especially if you are on your feet for a good part of the day. A few tips to help reduce the swelling are:-
When sitting do not cross your legs/ankles.
Exercise your feet as often as you can by scrunching up your toes. Making circles with your feet.
If sitting for long periods of time make sure that you go for a short walk every hour or so.
You could even raise the foot of your bed a little so that when you are sleeping your feet are higher than your heart.